joint is defined as a connection between two bones in the skeletal system. Joints can be classified by the type of tissue present or by the degree of movement permitted.

Classification by Type of Tissue

Fibrous Joints

A fibrous joint is where the bones are bound by tough, fibrous tissue. These joints typically require strength and stability over a range of movements.

1. Sutures: Immovable joints (synarthrosis) found between the flat, plate-like bones of the skull. They allow limited movement until about 20 years of age, after which they become fixed and immobile. These joints are important during birth, as they allow deformation of the skull. Fig 1: Bones of the calvarium and cranial base
2. Syndesmoses: Slightly movable joints (amphiarthroses) comprised of bones held together by an interosseous membrane. Examples include the middle radioulnar joint and the middle tibiofibular joint.
3. Gomphoses: Immovable joints where the teeth articulate with their sockets in the maxilla (upper teeth) or the mandible (lower teeth). The tooth is bound into its socket by the strong periodontal ligament.

Cartilaginous Joints

In a cartilaginous joint, the bones are united by fibrocartilage or hyaline cartilage. There are two main types:

• Synchondroses: The bones are connected by hyaline cartilage and are immovable (synarthrosis). An example is the joint between the diaphysis and epiphysis of a growing long bone.
• Symphyses: These joints are where the bones are united by a layer of fibrocartilage and are slightly movable (amphiarthrosis). Examples include the pubic symphysis and the joints between vertebral bodies. Fig 2: Adjacent vertebral bodies connected by fibrocartilage: an example of a symphysis

Synovial Joints

A synovial joint is defined by the presence of a fluid-filled joint cavity contained within a fibrous capsule. These joints are freely movable (diarthrosis) and are the most common type of joint found in the body. They can be sub-classified into several types based on the shape of their articular surfaces and the movements permitted:

• Hinge: Permits movement in one plane, usually flexion and extension (e.g., elbow joint, ankle joint, knee joint).
• Saddle: Named due to its resemblance to a saddle on a horse’s back. Characterized by opposing articular surfaces with a reciprocal concave-convex shape (e.g., carpometacarpal joints).
• Plane: The articular surfaces are relatively flat, allowing the bones to glide over one another (e.g., acromioclavicular joint, subtalar joint).
• Pivot: Allows for rotation only, formed by a central bony pivot surrounded by a bony ligamentous ring (e.g., proximal and distal radioulnar joints, atlantoaxial joint).
• Condyloid: Contains a convex surface that articulates with a concave elliptical cavity, also known as ellipsoid joints (e.g., wrist joint, metacarpophalangeal joint, metatarsophalangeal joint).
• Ball and Socket: The ball-shaped surface of one rounded bone fits into the cup-like depression of another bone, permitting free movement in numerous axes (e.g., hip joint, shoulder joint).

Classification by Degree of Movement

• Synarthrosis: Immovable joints.
• Amphiarthrosis: Slightly movable joints.
• Diarthrosis: Freely movable joints.

This classification provides a comprehensive understanding of the types and functions of joints in the human body, essential for studying skeletal anatomy and movement.